Implantable cardiac stimulating devices capable of emitting back-up pulses to a heart ventricle when no evoked to a previously applied ventricular pacing pulse are known in the art, see U.S. Pat. No. 5,476,487.
This type of cardiac stimulating devices is generally denoted as autocapture pacemakers in the art. The autocapture pacemakers operate by sensing whether a response occurs in the ventricle shortly after a ventricular pacing pulse has been delivered, such as 5-20 ms after delivery. The sensing may, for instance, take place during 40-60 ms. If a response is sensed during this time interval then no back-up pulse is emitted. However, if no evoked response is sensed, the pacemaker emits a back-up pulse. The back-up pulse is usually emitted with increased output energy or power in order to secure capture of the heart. For example, a normal value for a ventricular pacing pulse could be an amplitude of about 1.5 V, whereas the back-up pulse can be emitted with an amplitude of about 4.5 V.
Sometimes a back-up pulse can be delivered to the ventricle during a phase of the heart cycle when a stimulating pulse is not wanted. Such unwanted pulses may be initiated, for instance, after the occurrence of a premature ventricular contraction (PVC). If such a PVC occurs more or less simultaneously with an atrial event, the PVC may be undersensed due to ventricular blanking after atrial stimulation. In such a case, a ventricular pacing pulse may be delivered in the refractory period after the PVC, during which the heart will not respond to the ventricular stimulation. No evoked response will consequently be detected and a back-up pulse is delivered. In an unfortunate case, the back-up pulse can be timed with the vulnerable period after the PVC, typically coincident with a part of the T-wave, during which a delivered stimulation pulse may induce repetitive rhythms, such as for example tachycardia or ventricular fibrillation (VF).
U.S. Pat. No. 6,952,609 discloses an implantable cardiac stimulating device capable of combating these problems of increased risk of unintentionally triggering tachycardia or VF due to unfortunate timing of back-up pulses. The implantable cardiac stimulating device has a control unit for controlling an atrial pulse generator dependent on an atrial sensing unit, in a first manner wherein no stimulating pulse is delivered to the atrium and in a second manner wherein stimulating pulses are delivered to the atrium. The control unit additionally prevents delivery of back-up pulses to the ventricle during a number of heart cycles when the control unit changes from the first manner of operation to the second manner of operation.